422 CHEMICAL ENGINEERING
for a few turns at the hot junction in order to give greater mechanical strength to the joint. The two wires are insulated by fireclay insulating tubes, or by asbestos sleeving or cord, and are connected to a suitable couple head forming the cold junc-.tion. For severe use it is necessary to encase the couple in a protecting tube of steel, chromel, porcelain, fireclay, etc. A different form of base-metal couple, known under the trade name of "pyod" consists of an outer tube of iron and an inner wire or rod of constantan. The two are fused at one end into a neat joint forming the hot junction and are insulated from each other up to the head or cold junction. The couple is thus mechanically stronger than couples formed of two wires and when used without an additional protecting tube, is somewhat less liable to contamination than the bare two-wire couple of the same type, since the constantan, which is probably more susceptible to contamination than the iron, is protected by the iron tube of the pyod. However, couples of this type must be further protected by outer tubes if subjected to severe furnace conditions. Couples of various shapes may be constructed for special purposes. Thus frequently it is desirable to make a right-angle bend in a couple. Usually a mounted couple will stand bending into various shapes but before bending one should examine the couple and temporarily remove porcelain tubes or insulators liable to be broken by the process.
The indicating instruments which are connected to the thermocouple are of three general types, those operating upon the galvanometric principle, as an ordinary voltmeter; those operating upon the potentiometric principle; and those operating upon a combination of these two principles. The first two types of instrument have been made automatically recording, as will be discussed elsewhere.
Galvanometer Method.—Galvanometers for measuring the electromotive force developed by a thermocouple usually operate on the d'Arsonval principle, having a moving coil mounted between the poles of a permanent magnet. The deflection of the pointer or boom attached to the coil is proportional to the thermoelectric current passing through the coil, and the current is proportional to the electromotive force developed by the couple. Different methods for mounting the coil are employed. The coil may be suspended both above and below by phosphor-bronze suspensions and although many foreign instruments of this type have proven delicate, in one of the latest forms of American instrument using this double suspension the galvanometer may be subjected to severe handling without any injury.
The scale of the instrument may be graduated to read electromotive force or temperature. By use of a series resistance mounted inside the galvanometer case and an extra binding-post terminal, two scale ranges may be obtained, one for base-metal couples of a certain type and the other for the rare-metal LeChatelier couple. For precision work, the scale should have a narrow mirror extending throughout its length for use in eliminating the error in reading due to parallax. While obtaining a reading the observer should use one eye only and he should move his head to the position where the pointer and its reflected image in the mirror coincide.
Indicators may be obtained in either the switchboard type or portable type. The former is desirable for permanent installations. Many switchboard instruments have a scale ruled on ground glass and illuminated from the rear by an electric lamp. The black-enameled graduations upon the scale thus appear distinct against a bright background. Paper scales illuminated from the front are usually employed. In general high precision is not required of an instrument of the switchboard type so that the graduations may be coarse and readable at some distance. However, it is possible to obtain nichlv accurate switchboard indicators in which case the scale